Technical Abstract: The activity of isolated Rubisco increased with temperature at both air- levels and at saturating CO2. In contrast, CER at both air and high levels of CO2 was inhibited by temperatures exceeding 32 and 40 C, respectively, under both photorespiratory (Pr) and non-Pr conditions. At air-levels of CO2, the activation state of Rubisco in leaves decreased as temperature increased, with no decrease in the capacity for RuBP regeneration. Rubisc inactivation was even greater when temperature was increased at elevated CO2. These data indicated that Rubisco activase could not maintain the activation state of Rubisco, even under high CO2, as temperature was increased. To determine if activase per se was the principle limitation in the system, we measured the temperature response of Rubisco activation by activase using isolated enzymes and saturating levels of ATP and RuBP. As temperature was increased, the ability of activase to maintain a high steady-state of Rubisco activation decreased in a manner dependent on the amount of activase added. In the absence of activase, Rubisco deactivated faster as temperature was increased. Our results indicate that the increased rate of Rubisco inactivation as temperature increased exceeded the capacity of activase to maintain a constant activation state. Thus, as temperature increases, the photosynthetic potential of leaves is limited by Rubisco activase even at high CO2.